Coaxial transmission line filter



March 3, 1953 P. l. RICHARDS` coAxAL TRANSMISSIN LINE FILTER zsHEE'rS-SHEET 1 Fild Jan. s. 194s /Nvsnron PAUL RICHARDS A TTORNE Y March 3, 1953 P. l. lcHARDs COAXIAL TRANSMISSION LINE FILTER 2 SHEETS- SHEET 2 Filed Jan. 5, 1946 FIG. 2

AIII'I nl IIIIIIIIIIIIIII l/VVE'NTR PAUL l. RICHARDS '/"3 el FREQUENCY ww )mL-u- ATTORNEY Patented Mar. 3, 1953 COAXIAL TRANSMISSION LINE FILTER Paul I. Richards, Cambridge, Mass., assigner to the United States of America as represented by the Secretary of War :Application January 3, 1946, Serial No. 638,899

1 Claim.

This invention relates generally to electrical apparatus and more particularly to an mde` rived type, coaxial band-pass filter.

In the mderived type filter the lattenuation of a signal within a pass band of frequencies between the cutoff frequencies is low while the attenuation on either side of the pass band rises sharply. The mderived type filter is also unique in that for two arbitrarily chosen frequencies in the suppression region, the attenuation may be made theoretically infinite.

The use of lumped-constant filters becomes more and more impractical as the frequency of operation is increased. It is an object of this invention to provide an mderived coaxial band-pass filter which may be used at relatively high radio frequencies. Still another object is to provide a band-pass filter wherein the attenuation rises sharply on either side of the pas-s band until it reaches an infinite value at two chosen frequencies. It is a further object to provide a coaxial band-pass filter which may be readily inserted or removed from a standard radio frequency transmission line.

Other objects, features and advantages of this invention will Vsuggest themselves to those skilled in the art and will become :apparent from the following Adescription of the invention taken in connection with the accompanying drawings in which:

Fig. 1 is a Icross-sectional View of a coaxial band-pass l-ter embodying the principles of this invention;

Fig. 2 is a schematic diagram of a lumpedconstant mderived band-p-ass filter;` and Fig. 3 is a typical -graph of the filter characteristics in which the attenuation of a signal input effected by the filter `is plotted :against a representative frequency range.

Referring specifi-cally to Fig. 1*, the filter consists of an -outer coaxial conductor I3, an vinner conductor III)` comprising two folded sections of transmission line effectively in ser-ies with each other, and two coaxialshunt stubs 2D and 2`I.

In the filter the outer diameter 4of a section of the inner conductor I Il is reduced to a size shown :approximately by center conductor II. The outer wall of the inner conductor IB is extended to form :a cylindrical conductor which is coaxial about said ycenter conductor II. Approximately at the midpoint of the center conductor section I I, vthe cylindrical conductor mentioned above is divided to form two separate sections I2 and I4, hereinafter referred to 2 as folded transmission line sections or, -more briefly, `folded. sections. The space between th'e cylindrical conductors will be referred to as the center gap I5.

The `first folded section of length L1 is formed by cylindrical conductor I2 coaxi-ally centered about center conductor II. Y folded section of length L2 is formed by the cylindrical conductor I4, also coax-ial about center conductor II although separated from the first folded section by Vcenter .gap I5.

Two coaxial shunt stub sections are Yform-ed by joining two coaxial lines l2li and 2| at right angles to the coaxial filter. Center conductor 22 of stub section 20 makes contact with cylindrical conductor I2 :and Ais located at apredetermined distance from the center gap lto be @described later. Center conductor '23 of stub-section 2| makes contact with cylindrical conductor I4 and is located at a `predetermined ldistance from the centergap to be described rlater.

The filter is termin-ated at the `end thereof with standard transmission line yconnectors 24 and 25 which enable the filter to be readily inserted or removed from a radio frequency transmission line.

The filter vdescribed above is analogous to the pi section mderived, lumped-constant fil- ;f' ter structure shown in Fig. 2. The lumped-constani-l mderived typefilter is well known to those skilled in the art `and may include two parallel tuned circuits 52 :and 53 connected in series between one input terminal 50 and one output terminal 54. Input terminal 50 and output terminal 54 are connected by parallel resonant circuits 56 and 51, respectively, to a common input-output terminal 5I.

The relative attenuation imposed by a typical m-derived, band-pass filter for a band of signal inputs of different frequenciesis shown by the graph in Fig. 3. The upper and lower cut- 01T frequencies are designated by f1 and fr respectively in Fig. 3. The pass band includes all those lfrequencies between the upper and lower cutoff frequencies where the attenuation of a signal input is relatively low or zero. The suppression region is included in a. band of frequencies below the lower cutoff frequency :and a band above the upper cutoff frequency where the attenuation of a sigrral input is relatively high.

In the m-derived type filter the attenuation of a signal input of frequency fa in the suppression band below the lower cutoff frequency,

The -second l and of a signal input of frequency f4 in the suppression band above the upper cutoff frequency theoretically is innite. The frequencies fa and f4 will hereinafter be referred to as the infinite attenuation frequencies.

In the design of the mderived type, coaxial band-pass filter shown in Fig. 1, the first folded transmission line section L1, which is formed by the concentric cylinder I2 and center conductor II, is one-fourth of the wave length of the lower infinite attenuation frequency, f3, in length. The second folded transmission line section Lz, which is formed by concentric cylinder I4 and center conductor II, is one-fourth wave length of the upper infinite attenuation frequency, f4, in length. The short-circuited ends of the folded sections are reiiected at the open ends of these sections as high impedances. Hence the folded sections appear as parallel tuned circuits in series with each other.

The length L3 of each of the shunt stub sections and 2| from the shorted end thereof to the center conductor II is approximately onefourth wave length long at the mid-frequency of the pass band. The short-circuited end of each of the shunt sections is reflected to the opposite end thereof as a high impedance, and hence these stubs appear as parallel tuned circuits shunted between the center conductor section and the outer conductor section of the coaxial filter,

The characteristic impedances of the shunt stubsections 20 and I2I and the folded line sections are determined by the cutoff frequency and the desired characteristic impedance of the filter as a whole. Let Z1 be the characteristic impedance of the first folded section L1, let Z2 be the characteristic impedance of the second folded section L2 and Z3 be the characteristic impedance of the coaxial shunt stub sections. Then the relation between the characteristic impedance of the folded sections and the cutoif frequencies will be given by the equation:

1 lil-cos 20 p -cos 20+cos 28a wn=cutoff frequency (upper or lower) and V=propagation velocity through the filter which is equal to approximately 3 1010 cnr/sec.

The relation between the characteristic impedance of the shunt stub-section as 2l) or 2I, the folded section as I2 or I4, and the overall filter characteristic impedance, K0, is given approximately by the equation:

Z1Z2=Ko2(1c0s 1ra) It has been found that in order to avoid detuning the series folded sections I2 and I4 due to the distributed capacitances of the shunt stubsections 2U and 2|, the shunt stub sections should make contact with the cylindrical conductors at a point approximately one-eighth wave length from the center gap I5.

A coaxia rnderived type band-pass filter of the type described above will provide satisfactory operation at relatively high radio frequencies, The attenuation in the suppression regions rises very rapidly on either side of the cutoff frequencies until it reaches an infinite value at the respective infinite attenuation frequencies.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention.

The invention claimed is:

A coaxial transmission line band-pass lter comprising an outer coaxial conductor, an inner conductor comprising a first folded-section transmission line consisting of a center conductor surrounded by a cylindrical conductor a quarter wavelength long at a frequency just outside one end of said band pass which is short-circuited to said center conductor at one end thereof and open at the other end thereof, said inner conductor also comprising a second folded-section transmission line consisting of a continuation of said center conductor surrounded by a second cylindrical conductor a quarter Wavelength long at a frequency just outside the other end of said band pass which is short-circuited to said center conductor at one end thereof and open at thev opposite end thereof, the open ends of said coaxial conductors of said first and second folded sections being adjacent and spaced to form a gap, and first and second shunt coaxial line stub sections, a quarter wavelength long at the center frequency of said band pass, coupled between said outer conductor and said iirst and second folded sections, respectively, at points spaced substantially one-eighth wavelength at the center frequency of said band pass from said gap.

PAUL I. RICHARDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,220,922 Trevor Nov. 12, 1940y 2,270,416 Cork Jan. 20, i942 2,284,529 Mason May 26, 1942 2,408,927 Gurewitsch Oct. 8, 1946 2,446,982 Pound Aug. 10, 1948 2,465,801 Gurewitsch Mar. 29, 1949 2,467,292 Burrows Apr. 12, 1949 2,470,805 Collard May 24, 1949 2,484,028 Hansen Oct. 11, 1949 

